The ex-situ and in-situ gas diffusivities of polymer electrolyte membrane fuel cell catalyst layer and contribution of primary pores, secondary pores, ionomer and water to the total oxygen diffusion resistance

Oxygen transfer resistance in catalyst layer (CL) of polymer electrolyte membrane fuel cells (PEMFCs) is the limiting factor under high current density operation. The gas diffusivity of the CL has been studied ex-situ and in-situ in the literature, however, an order of magnitude difference between values of ex-situ and in-situ diffusivities and contribution of pores, ionomer and water to the total mass transfer are yet remain to be investigated. In this study ex-situ and in-situ gas diffusion of ten different CL designs are measured. The ex-situ measured resistance is a part of the total resistance measured in-situ, and having that, the constituting parts of total diffusion may be obtained. The diffusion resistance of ionomer-water accounts for about 80% of the total resistance, and the rest is almost all due to secondary pores. Although, primary pores' direct contribution to the diffusion resistance is negligible, their size has a notable impact on the ionomer-water relative diffusivity and increasing their diameter can linearly increase the ionomer-water diffusivity. Moreover, porosity of CL affects ionomer-water diffusivity by power of three. This study shows the efforts to reduce the total oxygen diffusion resistance should focus on increasing the CL porosity and size of primary pores.